1. Field of the Invention
This invention relates to automatic sampling apparatus for analytical equipment and, in particular, to sampling apparatus for flameless atomic absorption spectrometers.
2. Description of the Prior Art
In the analysis of samples by means of atomic absorption spectroscopy, it is necessary that the sample, in liquid form, be rendered in an atomic state, i.e., to form what is often referred to in the art as an "atomic cloud". A beam of radiation of the resonance spectral line of the element to be detected is then passed through the atomic cloud to a suitable detector. As originally conceived, the atomization was carried out by spraying a nebulized vapor of the liquid sample into an open flame. In recent years, however, there has been increasing use of flameless atomization which has certain technical advantages particularly for certain elements. The most common form of flameless atomizers is a heated graphite furnace which typically takes the form of a graphite sample tube which is electrically heated by passing an electric current between electrodes disposed at the respective ends of the tube. The sample is introduced through a radial port in the wall of the tube and the spectral beam passed longitudinally through the tube.
The electrical heating current is programmed to increase in three stages so as to effect respectively drying, ashing and atomization of the sample.
At the present time, liquid samples are normally introduced manually into the sample port by means of a syringe or micro-pipette. The programmed heating of the tube is then carried out, usually automatically by means of a suitable programmable control unit: each measurement requires approximately 1 to 2 minutes. Because of the necessary manual introduction of a new sample after each individual measurement, flameless AA analysis is rather time consuming.
Whether sample introducton is effected manually or automatically, two basic requirements must be met in carrying out a series of analyses: (1) sample carryover must be avoided, i.e., the sample involved in one specific analysis must not be involved with the succeeding analysis; and (2) in view of the extremely high sensitivity of atomic absorption spectroscopy, special care must be taken to prevent contamination by dust, separating agent residues, or the like with the resultant falsification of the analytical results.
In the present state of the art, the problem of carryover is solved by the use of disposable micro-pipettes which are replaced after each sample. While this solution may be acceptable for manual sample introduction, its automatation would involve an undesirable, and perhaps unacceptably high, degree of complexity. Moreover, replacement of the micro-pipette tip does nothing to alleviate contamination which continues to be a problem.